Semiconductor transistors, in particular field-effect controlled switching devices such as Metal Oxide Semiconductor Field Effect Transistors (MOSFET) and Insulated Gate Bipolar Transistors (IGBT) have been used in a wide variety of applications such as power supplies, power converters, electric cars and air-conditioners. Many of these applications are high power applications, which require the transistors to be able to accommodate substantial current and/or voltage, e.g., voltages in the range of 200V, 400V, 600V or more. In high power applications, two device parameters that play a substantial role in overall performance of the device are on-state resistance (RON) and breakdown voltage (VBR). Lower on-state resistance RON is a desirable characteristic because it minimizes the resistive power loss and corresponding heat generation that occurs when the device is in a forward conducting state. Meanwhile, high breakdown voltage VBR is a desirable characteristic because it determines how much voltage the device can safely block in an OFF state.
Power transistors typically include a lightly doped drift region between the output regions (e.g., source/drain regions) that substantially determines the breakdown voltage of the device. In the case of a vertical switching device (i.e., a device that is configured to conduct between opposite facing main and rear surfaces of the substrate), the drift region occupies most of the thickness of the substrate. The properties of the drift region can be tailored to achieve a desired tradeoff between on-state resistance and breakdown voltage. For example, by reducing the doping concentration of the drift region, the breakdown voltage the device can be improved. However, this comes at the expense of increased on-state resistance RON. Conversely, the doping concentration of the drift region can be increased to lower the on-state resistance at the expense of a reduced breakdown voltage VBR.
Field electrodes are used in power switching devices to favorably shift the tradeoff between on-state resistance and breakdown voltage. Field electrodes utilize the compensation principle to balance charges during operation of the device. By tying the field electrode to a fixed potential (e.g., source potential) during the OFF state of the device, charges in the drift region are compensated for by corresponding charges in the field electrode. This charge balancing technique makes the device less susceptible to avalanche breakdown than would otherwise be the case in the absence of a field electrode. As a result the doping concentration of the drift region can be increased, and thus the on-state resistance of the device reduced, without detrimentally impacting the voltage blocking capability of the device.
In one power semiconductor device configuration, field electrodes are provided in so-called “needle trenches.” An example of a needle-trench-configured device can be found in U.S. Pat. No. 8,247,865 to Hirler, which is incorporated herein by reference in its entirety. In general, “needle trench” style devices are vertical trenched-gate devices in which the trenches that contain the field electrode have a circular shape (from a plan-view perspective of the substrate) and are disposed at regular intervals along the length of the gate trenches. One advantage of these “needle trench” devices is improved scalability, i.e., miniaturization, in comparison to other trench designs. However, current techniques for forming the needle trenches introduce require many processing steps, which introduces additional time and expense into to the fabrication process.